A process for refining metal surfaces is described and claimed in Michaud et al U.S. Pat. No. 4,491,500, which process involves the development, physical removal and which continuous repair of a relatively soft oxide coating on the surface. High points are removed through mechanical action, preferably generated in but not limited to vibratory mass finishing apparatus, and very smooth and level surfaces are ultimately produced in relatively brief periods of time. Although the process described in the patent is most effective and satisfactory, it is self-evident that the realization of even higher production rates would constitute a valuable advance in the art.
Among the active ingredients specified in the Michaud et al patent, to provide exemplary formulations, are a phosphate salt or acid, or a mixture thereof with oxalic acid, sodium oxalate, or the like. The incorporation of a sulfate or chromate compound is disclosed, as is the use of metal phosphate activators or accelerators, and organic and inorganic oxidizers; if used, the latter is included in a minimum amount of 0.5 percent by weight of the total liquid substance. As a specific working example the patentees disclose a solution consisting of eight ounces, per gallon of water, of a mixture of 15 percent of sodium tripolyphosphate and 85 percent oxalic acid, to which is added 1.0 percent, based upon the total weight of the liquid substance, of a 35 percent aqueous solution of hydrogen peroxide (i.e., 0.103 gram mole per liter) containing a small amount of phosphoric acid stabilizer.
In such formulations, it is known that the incorporation of an oxidizing agent, e.g., hydrogen peroxide, produces a substantial increase in activity. This result is however accompanied by significant detrimental side-effects. In particular, as employed in the prior art the peroxide causes considerable dissolution of the metal, giving rise to very difficult problems of dimensional control and accuracy.
For example, oxalic acid/hydrogen peroxide solutions have been utilized widely for the surface refinement of workpieces prior to electroplating. Because of the tendency for the solution to dissolve metal from the non-contact surfaces, it has been necessary to form the workpiece with extra thicknesses of metal in such areas, so as to accommodate the dissolution and to endeavor to thereby produce the ultimate dimensional specifications. Such a process is obviously difficult to control and is, at least to that extent, inherently undesirable. (As used herein, the terms "non-contact surface" or "non-contact area" refer to those surfaces of the workpiece that are not exposed to substantial contact, during the surface refinement operation, by other workpieces or any mechanical finishing media used, such as are present within the open or box-end of a wrench.)
Furthermore, the peroxide concentrations heretofor employed have had a tendency to produce pitting of the metal surface, in turn creating a flawed appearance on the finished article. This gives rise to the need for further refinement of the surface, or necessitates the acceptance of a product of inferior surface quality.
Accordingly, it is an object of the present invention to provide a novel solution, and a novel composition for producing the same, which is highly effective for the refinement of metal surfaces utilizing a chemical/mechanical finishing technique.
It is a more specific object of the invention to provide such a solution and composition by which the surface refinement is achieved at an increased rate, while avoiding pitting of the surface or a substantial dimensional decrease on non-contact surfaces.
An additional specific object of the invention is to provide a solution and composition having the foregoing characteristics and advantages, which are of such chemical activity that the conversion coating produced is reformed continuously and at a high rate, and is of increased thickness, both features enabling utilization of mass finishing apparatus at high energy levels and thereby maximizing production rates.
A further object of the invention is to provide a novel process for the refinement of metal surfaces utilizing such solutions, which process achieves the desired surface and dimensional characteristics at high production rates, and is adapted to be carried out under ambient conditions.